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Wheat chromosomes controlling regular bipolar segregation
of homologous chromosomes and integrity of the centromere B. C. JOSHI, R. N. SAWHNEY, D. SINGH and Sushil KUMAR Division of Genetics, Indian Agricultural Research Institute, New Delhi, India The monosomic series of wheat varieties Chinese Spring and Pb. C591 are being maintained at the Division of Genetics, Indian Agricultural Research Institute, New Delhi (210 metres above sea level) and the Wheat Breeding Station, Simla (2120 metres above sea level). In the present study data were compiled on the number of different types of plants that are recovered from among the progenies of monosomics and their crosses from the year 1961 to 1968. These data are presented in Table 1. Column 4, denoting other aneuploids, includes plants with isochromosomes and telocentric chromosomes in addition to the background chromosome complement of 42, 41, 40 and other aneuploid numbers. Frequency of double monosomics: In the crosses involving Chinese Spring monosomics and varieties Pb. C591 (Delhi) and E5883 (Simla) the frequency of double monosomes has been recorded as 6.1 and 4.1 percent respectively (Table 1). These frequencies are significantly higher than those obtained by selfing Chinese Spring and Pb. C591 monosomes under the same experimental conditions. From different year's data it has been observed that a majority of double monosomes (54 percent) originate from monosomics for chromosomes 1B, 4B and 6B. Significantly, out of these chromosomes 1B and 6B are satellited. Frequency of other aneuploids: It is observed from the table that the progenies of Chinese Spring and Pb. C591 monosomics which possess isochromosomes and telocentric chromosomes have a much lower frequency (range 1.0 to 1.9 percent) than the segregates derived from the crosses of these monosomic series (range 2.2 to 12.5 percent) with other varieties of wheat. It was noted that 84.6 percent plants which showed isochromosomes and telocentrics were derived from monosomic lines for chromosomes 1B, 4B, 2D and 4D. The data presented reveal that chromosomes 1B, 4B and 6B of Chinese Spring and Pb. C591 control regular bipolar segregation of homologous chromosomes in each of the bivalent. In the absence of single dose (hemizygous state) of these chromosomes more double monosomes are produced. It is expected that the chances of "univalent shift" in the monosomic plants for these chromosomes would be higher. Regarding the production of plants possessing isochromosomes/telocentrics it has been observed that chromosomes 1B, 4B, 2D and 4D are involved in maintaining the integrity of chromosome at the centromere. Their absence in one dose promotes misdivision of the centromere, leading to the formation of isochromosomes or telocentrics. Significantly, absence of chromosomes 1B and 4B promotes the production of both the double monosomes and that of isochromosomes and telocentrics. Results of this study suggest that the regular bipolar segregation of homologous chromosomes and the integrity of the centromere are under genetic control and that chromosomes 1B, 4B, 6B, 2D and 4D are involved in these processes. (Receive January 27, 1970) |